(CNN) -- Missouri authorities have identified additional people killed in last weekend's tornado, bringing the toll of the United States' single deadliest twister ever recorded to 142 deaths, authorities said Saturday.

Among the dead was Will Norton, a recent high school graduate who attracted national attention when he disappeared during the storm.

The 18-year-old was driving home from his graduation Sunday when the tornado destroyed the Hummer H3 he and his father were in.

"Mark (Norton's father) said that he reached over and he grabbed Will with both of his arms ... he held on to him until he possibly couldn't anymore and so he's feeling really bad about that because as a dad you don't want to ever let go of your kids. You want to protect them forever. But at least we know that he did absolutely everything he possibly could," said Tracey Presslor, Norton's aunt, through tears.

Norton's body was found Friday by divers in a pond close to where his vehicle had been.

Meanwhile, 100 persons remain unaccounted for, the Missouri Department of Public Safety said in a statement Saturday.

Authorities are relying on a scientific identification of the remains, and while that process is slower, it's more reliable than a family member's visual identification, she said.

Authorities are using past X-rays, dental records and body markings to help identify bodies, Spillars said.

"We will go through this process as quickly as possible, knowing how important it is to be accurate," she said, joined by law officers and others behind her. "We know that this has been a community that has been tragically impacted by this. All these people behind me have worked 24 hours a day.

"It's a very scientific process. It's a very exacting process," Spillars continued. "I can tell you that it's a very respectful process, and I can tell you that someone is with that loved one every step of the way."

More than 50 state troopers are working around the clock on the cases, she added.

Many anxious family members have been waiting to hear what became of missing loved ones in Joplin, the flash point of a wicked tornado last Sunday that has produced the highest death toll from a single tornado in the United States since modern recordkeeping began in 1950.

Joplin City Manager Mark Rohr told reporters Saturday that the death toll has reached 142, up from 132 reported a day earlier.

"We're still in search-and-rescue mode," he said, adding that more than 500 search-and-rescue volunteers were in Joplin Saturday working.

As many as 3,000 volunteers are thought to be in the city in total, he said, helping in a variety of ways.

Roving groups of people with tools and equipment were spotted going door to door -- or lot to lot -- asking what they could do.

Volunteers arrived in cars and vans, pickups and trucks -- many with out-of-state license plates. Some painted their vehicles to announce their intentions. Signs read, "Joplin Bound, "From Plano TX to Joplin" and "Free Chainsaw Work."

The number of people missing from the massive tornado has steadily fallen to 100 -- from 156 on Friday and 232 on Thursday, officials said. Included in the 100 still missing are nine people who were reported dead by their families, but for whom official confirmation is underway.

Still, residents have expressed angst over the time-consuming identification process.

"It's frustrating to the families and it's frustrating to me," Newton County Coroner Mark Bridges said this week. "It just takes time to go through those identifiers and get them to the families."

As of Friday morning, more than 2,500 Missourians affected by Sunday's tornadoes have applied for federal assistance to help with home repairs and cover other personal losses, according to the Federal Emergency Management Agency.

The agency reported that more than $2.8 million worth of assistance has already been approved, which is separate from public assistance that the state can receive for emergency response needs and longer-term rebuilding projects like schools, roads and firehouses.

President Barack Obama is expected to tour Joplin on Sunday, meeting with state and local authorities as well as families affected by the devastation. A moment of silence is expected to be observed Sunday too.